1. Field of the Invention
The present invention relates to an optical add/drop device.
2. Description of the Related Art
With a recent increase in traffic, a large-capacity network has been required. To meet this requirement, the construction of a WDM (wavelength division multiplexing)-based optical network will be necessary in a future trunk network. An optical add/drop device (optical ADM: optical add/drop multiplexer) capable of adding and/or dropping optical signals having different wavelengths by the wavelength between this device and another network device such as a router and an ATM (asynchronous transfer mode) switch is used in an optical network. It is important to form a ring network by using this optical ADM and to form an optical network tolerant of failure.
FIG. 1 is a block diagram of a general optical add/drop device in the prior art. An optical add/drop device 2 is arranged along an optical transmission line 4 for transmitting WDM signal light obtained by wavelength division multiplexing a plurality of optical signals having different wavelengths λ1 to λn (n is an integer satisfying 1<n). This device 2 has an optical demultiplexer 6 for separating the WDM signal light supplied from the input-side optical transmission line 4 into the optical signals having the wavelengths λ1 to λn, n 2×2 optical switches 8 for selecting optical signals to be passed through this device 2 and optical signals to be added/dropped, and an optical multiplexer 10 for wavelength division multiplexing optical signals to be output to the output-side optical transmission line 4.
k (k is an integer satisfying 1<k) regenerators 14 each having a wavelength conversion function are provided for an interface between this device 2 and another network device for an add signal, such as a router 12, and k regenerators 18 each having a wavelength conversion function are provided for an interface between this device 2 and another network device for a drop signal, such as an ATM switch 16.
Examples of each optical switch include a waveguide switch using a thermooptic effect and a mechanical switch having a motor. Examples of a wavelength converter usable as each regenerator include an all-optical wavelength converter using a semiconductor optical amplifier and a wavelength converter using opto/electrical conversion and electro/optical conversion. Examples of the optical multiplexer or the optical demultiplexer include a device using an arrayed waveguide grating (AWG) and a device using a dielectric multilayer film.
A device controller 20 is provided in the optical add/drop device 2 to perform setting or the like for signal adding, dropping, or passing according to instructions from an operation system 22 for supervising and controlling the whole of an optical network. In this manner, the optical add/drop device 2 selects signal adding, dropping, or passing to thereby allow a flexible operation of the optical network.
In the device shown in FIG. 1, an optical signal from the router 12 is wavelength converted by each regenerator 14 and accommodated into the optical network at the wavelength assigned to an input port of the optical multiplexer 10. That is, wavelengths are fixedly assigned in the optical network. For more flexible and efficient operation of the optical network, it is desirable that an optical signal can be accommodated into the optical network at an arbitrary wavelength.
FIG. 2 is a block diagram of an improved optical add/drop device in the prior art. In this device, k tunable wavelength converters 28 are used in place of the k regenerators 14 shown in FIG. 1. Further, a k×n optical switch 24 is provided between the tunable wavelength converters 28 and the optical switches 8, and an n×k optical switch 26 is provided between the regenerators (wavelength converters) 18 and the optical switches 8. The optical switches 24 and 26 may be replaced by AWGs. The number k represents the number of ports of an intraoffice interface between this device and another network device such as a router 12 and an ATM switch 16, and the number n represents the number of WDM channels. Accordingly, k□n in general.
In the improved optical add/drop device mentioned above, an optical signal from the router 12 can be wavelength converted to be added to an arbitrary wavelength channel owing to the use of the tunable wavelength converters 28 and the optical switch 24. In case of converting the wavelength of an optical signal to be added into a wavelength λn, the optical switch 24 is controlled to perform routing so that the optical signal from the router 12 is passed through the optical switch 8 connected to the input port of the optical multiplexer 10 corresponding to the wavelength λn.
In the prior art, the optical signals passing through the optical add/drop device are not subjected to wavelength conversion. Accordingly, a function of waveform shaping or the like associated with signal regeneration processing cannot be obtained, so that long-haul transmission is difficult to attain.
Further, in the prior art, a tunable wavelength converter including a tunable light source is necessary at an interface between the optical add/drop device and another network device such as a router and an ATM switch, so as to add a signal from the other network device to an arbitrary wavelength channel in the optical network.
Referring to FIGS. 3A and 3B, there are shown different tunable light sources in the prior art. In the configuration shown in FIG. 3A, a plurality of laser diodes (LD1 to LDn) 30 are kept steadily driven, and CW light (DC light or unmodulated light) output from one of the laser diodes 30 selected by an n×1 optical switch 32 is modulated by an optical modulator (Mod) 34 to obtain an optical signal having a desired wavelength. An optical signal from another network device is converted into an electrical signal by an opto/electrical (O/E) converter 36, and this electrical signal is supplied as a modulating signal to the optical modulator 34. The optical switch 32 is controlled by a device controller 38.
In this configuration, the time for switching wavelengths depends on the transit time of the optical switch 32. Further, there is a possibility of crosstalk in the optical switch 32 and an increase in cost and power consumption.
In the configuration shown in FIG. 3B, a plurality of drive circuits (DRV) 42 respectively associated with a plurality of laser diodes 40 are turned on/off by a device controller 44 to select one of the laser diodes 40 and steadily drive it. CW light output from the selected laser diode 40 is modulated by an optical modulator 48 to thereby obtain an optical signal having a desired wavelength. An optical signal from another network device is converted into an electrical signal by an opto/electrical converter 50, and this electrical signal is supplied as a modulating signal to the optical modulator 48. The plural laser diodes 40 and the optical modulator 48 are optically connected by an AWG 46.
Also in this configuration, the LDs and the DRVs whose number corresponds to the number of wavelengths for each are required as similar to the previous configuration shown in FIG. 3A, causing an increase in cost.
Thus, a tunable light source is required in the prior art, so as to add a signal from another network device to an arbitrary wavelength channel in the optical network, resulting in an increase in cost of the optical add/drop device.